The invention relates to a low-pressure mercury vapour discharge lamp for suntanning purposes comprising a closed tubular discharge vessel having an at least substantially circular cross-section, which discharge vessel contains mercury and a rare gas and is provided at the inside with a first luminescent layer which comprises a first luminescent material which emits mainly UVA and with a second luminescent layer which comprises a second luminescent material which emits mainly UVA, the first and second luminescent materials also showing UVB emission with mutually differing relative quantities of UVB radiation energy.
Such a lamp is known from the European Patent Application 0 228 737.
In the known lamp, a first luminescent layer is present over the entire circumference of the discharge vessel, which layer comprises lead-activated barium disilicate (BSP) or cerium-magnesium aluminate (CAM) as the luminescent material. A second luminescent layer is provided on the first luminescent layer at the discharge side and over the entire circumference of the discharge vessel, which second layer comprises europium-activated strontium metaborate (SBE) as the luminescent material.
The SBE has a relatively lower UVB emission (UVB energy calculated as a percentage of the quantity of UVA radiation energy emitted) than the BSP.
It is known that both UVA radiation (315-400 nm) and UVB radiation (280-315 nm) play an important part in the pigmentation process in the human skin. The main interest here concerns the process which is known as indirect or delayed pigmentation and which leads to a more permanent browning of the skin. UVB radiation is more effective for this process than is UVA radiation. A large dose of UVB radiation, however, is undesirable since it can lead to erythema (sunburn). Seen in this light, it is interesting to have available the possibility of adapting the UVB-radiation content to the wishes of the users.
In the known low-pressure mercury vapour discharge lamp mentioned above the UVB-radiation content is fixed. It is true that one could use two lamps of mutually differing UVB contents, but this means that extra provisions are necessary for switching the two lamps on and off separately. In solariums in which several lamps are present, moreover, this leads to a drop in the radiation level (W/m.sup.2) since part of the lamps would then be switched off.